The present invention relates generally to the field of commercial and industrial power generation, and in particular to a new and useful injector for gas-fired burners used in furnaces.
There are four main methods for firing natural gas in large-scale, commercial fossil fueled burners. Two of these methods, depicted in FIGS. 1A and 1B, contemplate a plurality of separate, equally spaced natural gas injection nozzles (spuds) mounted in the swirling air zones near the burner exit. In a third method, multiple longitudinal spuds are arranged in a circular array, as shown in FIG. 1C. As can be seen in FIGS. 1A, 1B and 1C, these three burner arrangements all employ a common manifold to distribute the natural gas among a multitude of gas spuds, adding a substantial level of undesirable complexity. The burner arrangements of FIGS. 1A and 1B also require the spuds to be placed in fixed positions in the air zones. Notably, each individual spud may have one or more holes drilled into it in order to direct gas flowing therethrough.
Undesirable complexity is further compounded in burners designed to fire both gas and pulverized coal, either alone or in combination, using multiple axial gas spuds. The spuds are difficult to fit in the burner without interference with the coal nozzle and elbow assembly, and are not easily retracted for protection from heat and slag during coal-only firing.
In contrast, the fourth arrangement, shown in FIG. 1D, employs a single gas injection nozzle, i.e. a single gas injector or “super spud”, positioned in the center of the burner. In this fourth arrangement, natural gas flows from a pipe into a large, single gas element and disperses through multiple holes drilled at its discharge tip. Known super spud elements do not impart any swirling component to the discharged gas, so that the natural gas is discharged from the injection holes with only axial and/or radial velocity components, i.e. without a tangential component to the velocity. FIGS. 4A and 5A illustrate such a prior art single gas injector 100a in which each injection hole 120 is formed straight through the end wall 170 of the injector 100a. The injection holes 120 are arranged equally spaced around a center hole 110.
Previous firing of natural gas at 100 million Btu/hr in a large-scale test facility via multiple spuds in a fossil fuel burner, similar to the arrangement shown in FIG. 1A, resulted in acceptably short or transparent flames, but very high NOx concentrations (>600 PPMV or 0.7 lb NO2/106 Btu). This arrangement did not include the use of flue gas recirculation (FGR) or overfire air (OFA). FGR is a proven NOx reduction technique in which a portion of flue gas from the boiler exit is mixed with the secondary air and introduced into burner. NOx levels can also be reduced by using OFA, which is a process where part of the combustion air is diverted from the burner and injected above the flame zone for substoichiometric burner operation.
Large-scale testing of a burner equipped with multiple axial spuds encircling the inner wall of a central core pipe, similar to the arrangement shown in FIG. 1C, achieved lower NOx concentrations (165 PPMV NOx or 0.18 lb NO2/106 Btu) compared to the arrangement of FIG. 1A, and produced 16 PPMV CO at 9% excess air and 100 million Btu/hr, again without FGR or OFA. Under these conditions the flame length was 24-26 feet. While these levels of NOx, CO and flame length are presently acceptable, this arrangement, as noted above, is mechanically complex.
Single element centerline, or super spud, firing of natural gas in three different fossil fuel burners, using an arrangement similar to FIG. 1D, generated 67 to 88 PPMV NOx (0.08 to 0.10 lb NO2/106 Btu) with luminous, 28 to 30 ft long flames at 100 million Btu/hr firing rate and 9% excess air level.
As demonstrated by the above results, single element centerline (super spud) gas firing, via the arrangement of FIG. 1D, seems to be more attractive than firing by multiple spuds, via the arrangement of FIG. 1C, due to low NOx emissions and mechanical simplicity. Nevertheless, despite the significantly lower NOx emissions, natural gas injection through a single centerline super spud has resulted in long flame lengths. These longer flames can impinge on the opposite walls of shallow depth boilers or furnaces, resulting in poor heat absorption and deterioration of combustion.
In contrast, the arrangements of FIGS. 1A and 1B, with spuds mounted in the swirling air zones, produce short or transparent flames, but with high NOx performance. Multiple longitudinal spuds in the burner core zone, the arrangement of FIG. 1C, generated higher NOx levels than the centerline super spuds, in addition to luminous flames exceeding the length of those observed for peripheral spuds.
Therefore, a better way of discharging natural gas and other gaseous fuels from fossil fuel burners is needed to reduce the flame length without significantly increasing the NOx emissions.